So, I didn't even know this was a Clematis. It is my mother's and she always told me it was a "Clementis" (so ignore the title of the picture because it is wrong). I don't know what kind it is though. It is growing up a trellis and is probably about 5+ feet tall right now. The flowers are a decent size, probably about 5 or 6 inches across. And from looking at pictures I think it is the closest to Clematis durandii...but it still didn't even look exactly like that. Any help would be appreciated thanks!

Central Massachusetts, Zone 5

Here is a link that might be useful: Purple Flower, please ID!